The recognition that rotavirus is an important etiologic agent of life-threatening infantile diarrheal disease has led to significant efforts to control the virus and to prevent the disease. Estes et al., "Rotavirus Antigens". In Atassi and Backrach eds., Immunobiology of proteins and peptides-III. Plenam New York p. 201-14 (1985); and Kapikian, A.Z., et al. In Fields, B.N., et al. (eds.), Virology, Raven Press, New York, p. 863-906 (1985). Although it is known that oral rehydration is an effective method for reducing diarrheal disease mortality, other interventions are needed to reduce morbidity and possibly eradicate this disease. Eradication of the disease would require immunization on a global population basis. This immunization could involve the live attenuated pathogens themselves or pathogen-specific antigenic proteins that induce neutralizing protection from disease (possibly mediated by antibodies). Elucidation and understanding of the rotavirus gene structure will greatly facilitate the efforts to eradicate the disease.
There are several problems associated with efficacy and safety when using live rotavirus vaccines:
(1) A vaccine that only produces a "mild" form of the disease may not provide safe and effective immunization in the gastrointestinal tract. Although viruses are relatively efficient in inducing resistance to subsequent infection, resistance induced by oral vaccination with attenuated virus strains may show more variability.
(2) There are significant risks that the attenuated virus will revert to virulence.
(3) Although the live vaccines may be efficious and safe when tested or used in children from developed countries, there is no guarantee that the same vaccine will be safe in children in developing countries. This is a serious concern since the susceptibility to diarrheal disease is enhanced in children in developing countries due to malnutrition and/or concurrent infections from other pathogens.
(4) Because of the expense of production and safety testing, the cost of distributing live vaccines in developing countries, where they are needed the most, may be prohibitive.
(5) Finally, in developing countries concurrent infections with multiple enteric pathogens may interfere with vaccine "takes".
The genome of rotavirus consists of 11 segments of double-stranded RNA. The genomic RNA is enclosed within a double-layered protein capsid that consists of the structural proteins VP1 to VP9. Estes, M.K. et al, "Rotavirus Antigens". In Atassi and Bachrach eds., Immunobiology of proteins and peptides-III. Plenum, New York p. 201-14 (1985). Each genome segment encodes at least one protein. Chan, W.K., et al., Virology 151:243-252, (1986), and Mason, B.B., et al., J. Virol, 46:413-423, 1983. The outer capsid protein VP3 functions as a viral hemagglutinin and also plays a role in inducing neutralizing antibodies. VP7 is an outer capsid glycoprotein which also induces neutralization antigen antibodies; VP7 is reportedly the all-attachment protein. The major capsid protein, VP6, is located on the inner capsid. This protein comprises greater than 80% of the protein mass of the viral particle and contains the subgroup antigen (S antigen VP6). Estes, M.K., et al., "Rotavirus Antigens" In M. Z. Atassi et al. (eds.) Immunology of Proteins and Peptides-III, Plenum, New York, pp. 201-214 (1985). The presence of VP6 on virus particles has been associated with viral polymerase activity. Bican, P., et al., J. Virol, 6:1113-1117 (1982). VP6 interacts with viral proteins during replication and assembly. Estes, M.K., et al., "Rotavirus Antigens" In M. Z. Atassi et al. (eds.) Immunology of Proteins and Peptides-III, Plenum, New York, pp 201-214 (1985). Although this interaction is not completely understood, it involves binding to RNA genomic segments or transcripts. VP6 possesses an oligomeric, possibly trimeric, confirmation. Gorziglia, M. C. et al., J. Gen. Virol, 66:1889-1900 (1985). Additionally, VP6 is the major protein detected in diagnostic enzyme-linked immunosorbent assays. Beards, G.M., et al., J. Clin. Microbiol, 19:248-254 (1984). Neutralizing and protective antibodies are produced to the outer capsid VP7 and VP3 antigens, but information on the role of other structural proteins, VP1, 2, 6, and 9, in inducing protection from infection is less clear. Furthermore, other gene products, for example, non-structural proteins (NS 35, 34, 28) are also synthesized by the rotavirus genome.
The expression vector system is from the insect baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV). AcNPV has a genome of ca. 130 kilobases of double-stranded, circular DNA and it is the most extensively studied baculovirus. Miller, L.K., pp. 203-274 (1981). AcNPV has a biphasic replication cycle and produces a different form of infectious virus during each phase. Between 10 and 24 h postinfection (p.i.), extracellular virus is produced by the budding of nucleocapsids through the cytoplasmic membrane. By 15 to 18 h p.i., nucleocapsids are enveloped within the nucleus and embedded in a paracrystalline protein matrix, which is formed from a single major protein called polyhedrin. In infected Spodoptera frugiperda (fall armyworm, Lepidoptera, Noctuidae) cells, AcNPV polyhedrin accumulates to high levels and constitutes 25% or more of the total protein mass in the cell; it may be synthesized in greater abundance than any other protein in a virus-infected eukaryotic cell.
Polyhedrin is encoded by the virus, and the gene has been mapped and sequenced. The presence or expression of the polyhedrin gene is not required for the production of infectious extracellular virus. Inactivation of the polyhedrin gene by deletion or by insertion results in mutants that do not produce occlusions in infected cells. These occlusion-negative viruses form plaques that are different from plaques produced by wild-type viruses, and this distinctive plaque morphology is useful as a means to screen for recombinant viruses.